1. Field of the Invention
The present invention relates to industrial fabrics and has particular relevance to fabrics on which non-woven materials may be formed by hydroentanglement and other formation techniques for the “Nonwovens” market sector and to papermakers' fabrics, such as forming fabrics, dryer fabrics and fabrics for use in the production of paper products using through-air drying (TAD) installations.
2. Description of the Related Art
TAD fabrics are conventionally used in the manufacture of paper towels, facial tissue, bathroom tissue, table napkins and the like.
U.S. Pat. Nos. 6,017,417 and 6,331,230; and Publication WO 01/44568 describe the manufacture of tissue and the like using through-air drying. Typically, in such processes, a slurry of cellulosic fibers is fed onto a forming fabric or between two forming fabrics, where the paper web is formed and partially dewatered before the web is transferred, often via a transfer fabric, to a TAD fabric for further water removal by way of one or multiple TAD units. The web is then fed by way of the TAD fabric to a presser roll where a nip is formed between the TAD fabric and a Yankee cylinder. Here the paper web transfers to the Yankee cylinder where further drying and creping takes place. In one variation of this process, the Yankee cylinder is removed, thus eliminating the pressing nip. In this case, the web is transferred from the TAD fabric to a further fabric.
It is conventional to spray a chemical release agent, e.g. silicone oil onto the TAD fabrics in order to provide good sheet release, whether it be to aid the transfer of the sheet on to another fabric or on to the Yankee cylinder, after exiting the presser roll nip. There are a number of potential problems associated with using a chemical release agent in the TAD process, two of these being that they are messy to utilize and very expensive.
TAD fabrics are flat-woven fabrics, which are spliced together. Adhesive is applied to the terminal ends in the joint area to provide supplementary strength and to keep these terminal ends in-plane. It has been found that when no adhesive is present, the chemical release agent tends to facilitate the process of allowing the terminal ends to relax under operating temperatures, which causes them to come out of plane of the fabric. Once out of plane, damage to, or rupture of, the sheet will inevitably occur and the seam will slowly fatigue until premature failure occurs. The use of adhesive in the joint area helps keep the terminal ends in place but adversely affects the porosity of the fabric at the joint, which can in turn have an unfavorable impact on the product quality and machine performance.
What is needed in the art is a seam that has the property of terminal end restraint without utilizing an adhesive.
Furthermore, chemical release agents have been found to accumulate on the fabric causing waste fiber to build-up and block the surface. This also affects the rate of drying and thus paper quality.
Probably the most critical problem, with the use of the chemical release agent, is the fact that it remains in the recycled white water system. Most modern paper machines tend to have closed water systems, and so the water that is removed from the cellulosic stock during the papermaking process and the reclaimed fabric shower water is collected, recycled and then reused as shower water and also to dilute the new cellulsoic stock. In the interim period, the water is stored in holding tanks and here the minute beads of chemical release agent coalesce into larger globules. It is extremely difficult to separate the chemical release agent from the water and the globules end up coating these tanks, which finally make their way back into the system. When the globules find their way into the cellulosic stock, there are potentially a number of problems, all of which result in a reduction of paper quality and machine operating efficiency.
The paper stock is a complex, charged system, with additives, such as cationic retention aids, added in order to ensure that all of the individual components of the stock bond together. When oil gets into this system it interferes with these charges and suppresses the effectiveness of the additives. This in turn leads to higher operating costs since additional amounts of additives are needed to achieve the desired sheet properties.
Another problem is that globules in the stock act as a debonder and reduce the sheet strength. Machine refining must be increased to compensate for the loss in sheet strength, which makes the sheet harder to dewater and/or dry and, in some cases, results in a loss of machine speed and/or output.
During manufacture the sheet side of conventional TAD fabrics is sanded, so as to increase the surface contact area of the fabric from between 6–12% to between 20–30%. This is required in order to ensure good transfer of the paper web, for example, from the TAD fabric to the Yankee cylinder and it also ensures good final sheet strength. The sanding process usually encourages the onset of micro-fibrillation of the yarn components on the paper-facing side, a problem that is accentuated through the use of high pressure showers. These fibrils eventually cause a reduction in the fabric's permeability which in turn leads to a poor drying profile and subsequently to a lower machine output.
TAD fabrics are conventionally made from polyester yarns, designed to improve their dry heat, hydrolysis and abrasion resistant properties. The operating environment on a TAD machine accelerates polymer degradation due to these phenomena, which ultimately causes fabric failure to occur. Fabric cleanliness is also an issue with conventional dryer and TAD fabrics in that dirt or so-called “stickies” tend to adhere to the fabric surface, which can cause holes in the sheet. Conventional TAD fabrics may also suffer from a lack of rigidity leading to cross-machine direction undulations in the fabric, particularly post the TAD cylinders. Occasionally, the undulations can be so severe as to cause irreversible localized folding of the fabric, necessitating its removal from the machine.
What is needed in the art is a fabric that reduces or eliminates the foregoing related problems.